P0866

TCM Communication Circuit High

Indicator

The PCM has detected a malfunction in the traction control module communication circuit. The traction control module communication circuit delivers data to the PCM and other controllers where it is utilized to calculate engine RPM, fuel delivery strategy, and engine timing, among other things. This code applies to vehicles equipped with electronic traction control systems only.

Code Set Parameters

If the PCM (and/or other controllers) fails to receive an input from the traction control module communication circuit a code will be stored and a malfunction indicator lamp may be illuminated. The traction control malfunction indicator may also be illuminated (or flashing) and the transmission may also be placed in limp-in mode by the PCM.

Symptoms

Symptoms may include failed traction control operation, a lack of traction in slippery driving conditions, an illuminated or flashing traction control lamp, harsh shifting, failure to shift, reduced fuel efficiency, service engine soon lamp illumination, and a stored code.

Common Causes

By far the most common cause of this code is from a defective traction control ON/OFF switch. Liquids spilled into the traction control switch are a very common cause of this type of failure.. Other causes may include open, shorted, corroded, or otherwise damaged wiring and/or electrical connectors. Technicians also report the presence of excessive corrosion in the sensor connector as the cause of this code being stored.

Common Misdiagnosis

The traction control module (sometimes integrated into either the powertrain control module or transmission control module) is frequently condemned in error when a defective switch, open circuits, or faulty connectors are at fault.

Diagnosis

CAN is the abbreviation for “controller area network.” The CAN represents a communication bus that allows multiple microcontrollers to communicate with one another without the need for a host computer

It is a message based protocol originally designed for automotive use

The CAN bus network is actually a complex conglomeration of wiring harnesses and connectors used as a pipeline of information shared between two or more automotive control modules

These controllers control virtually every electrical function of the vehicle, with the PCM being the primary controller

Control modules receive input data from various sensors and emit output signals to system components and other control modules

For example: The PCM receives an input signal from the vehicle speed sensor

This signal is systematically forwarded to affected control modules that utilize the data to accomplish various other tasks

The cruise control module uses the data for speed control purposes, the traction control system uses the data to regulate vehicle traction control strategy, and the anti lock brake controller compares vehicle speed from the PCM with data inputs from individual wheel speed sensors to operate the anti lock braking system.
Several tools will be instrumental in attempting to successfully diagnose this code

A suitable OBD-II scanner (or code reader) and a digital volt/ohmmeter will be most helpful in trying to perform a successful diagnosis

Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data

This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored

Continue by clearing the code and operating the vehicle to see if it returns

This will help to determine whether or not the malfunction is intermittent

After the codes are cleared, test drive the vehicle to see if the code returns

If the code fails to immediately return, you may have an intermittent condition

Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can even be attempted

Unlike other diagnostic codes, this type of code can sometimes be best left to a professional simply because of the bulk of circuitry involved

An experienced technician with a specialized scanner (Autohex or Tech II) may be able to determine the general area of the malfunction much more rapidly and easily than someone using a code reader and a digital volt ohmmeter

Disconnecting and testing every single pin of the CAN bus could prove to be extremely time and cost prohibitive

Additionally, some type of memory saving device must be installed, lest the PCM and other controllers lose their memory and require reprogramming

A specialized diagnostic CAN scanner will show pin values and control module operation without risking a meltdown

It can accurately diagnose computer and circuitry problems by monitoring vehicle operation while the vehicle is being operated

To diagnose this type of code using a digital volt ohmmeter would entail probing thousands of circuits, independently

One misplaced probe could destroy expensive control modules and require that the vehicle be totally reprogrammed.
At the most, you may attempt to perform a continuity test after all control modules are disconnected, and this could literally require 40-hours or more, depending upon the vehicle

Some applications are equipped with up to 18 separate control modules.
If you choose to tackle this monumental task, begin with a careful visual inspection of all system circuitry, connectors, and fuses

Control module ground circuits should be tested for continuity with battery ground

These types of codes are frequently caused by defective or disconnected system grounds

An auxiliary ground cable can be helpful in diagnosing system ground discrepancies

Engine and transmission ground cables, straps, and wires are sometimes left dangling after repairs are performed

Look for loose or corroded electrical connectors that may increase circuit resistance and cause these types of codes to be stored.
Obtain a CAN bus system wiring diagram and/or pin out value chart, then use the digital volt ohmmeter to test continuity between individual controller connectors

Compare your findings with the manufacturer’s referenced values and repair open or shorted circuits as required

It is often much more frugal to replace defective wiring rather than attempting to remove it from the complex web of wiring harnesses